The present invention relates to variable ratio drives, and in particular to drive systems generally comprising two pulleys spaced one from another with an endless flexible member extending therebetween for transferring motion from one pulley to the next and further wherein at least one of the pulleys can have its diameter selectably changed.
Drive systems incorporating two spaced apart pulleys interconnected by an endless flexible member are quite common and are often used to translate rotational motion from one member to another. By selecting differently sized pulleys, the translated rotational rate can be either increased or decreased depending on the pulley ratio selected. In some applications, most commonly as found on bicycles, a range of operating ratios is desired to provide the user with a variety of rotational rate transfers to optimize the application of power to speed. The most common of these systems incorporates a driving pulley or sprocket which may be one or more coaxially stacked pulleys and which by means of the endless flexible member translates the rotational motion to a second set of pulleys or sprockets of different but fixed diameters and also sequentially stacked.
While this type of system is quite common and has achieved wide ranging use, it can be noisy and the manner in which speeds are changed is accomplished by laterally forcing the chain or belt from one diameter sprocket to a laterally adjacent and coaxial different diameter sprocket. Depending upon the desired ratio, it is quite common for the endless flexible member to be engaged upon pulleys or sprockets which do not lie in the same plane. Such a skewed operation of the flexible member can result in premature wear, excessive noise, and unwanted gear ratio changes.
Various attempts have been made to develop improved drive systems which address the aforementioned problems. Some of these attempts include spaced cones with a flexible member between each cone, or single pulleys having a variable diameter by the mechanical repositioning and retention of various elements or pines. However, these attempts have either failed to solve the aforementioned problems, or were too complex and expensive to provide an efficient and cost effective drive system. Thus, there has remained a need for a reliable and efficient variable ratio drive system.